This invention relates to magnetic field responsive devices and is more particularly concerned with a magnetic field responsive device having improved sensitivity and performance and including one or more giant magneto resistive (GMR) materials.
A variety of magnetic field sensors have been proposed hitherto. The simplest of these are Hall Effect magnetometers in which the size of the magnetic field is derived from measurement of a transverse voltage in a current-carrying semiconductor placed in the magnetic field. Such devices are exceedingly temperature sesitive, and in fact the measured values are exponentially dependent upon temperature. Classical magneto resistive materials, such as nickel iron, have also been used for stray field pick up, but such devices are too insensitive for many applicaitons.
Flux-gate magnetometers are also used to measure magnetic fields. In these devices a soft magnetic material is excited with an AC excitation field, with the response, as modified by the ambient field, being observed.
Layered sandwich-type magneto resistive devices comprising at least two deposited layers of magnetic thin films separated by a non-magnetic thin film layer have been proposed for use as magnetic memory cells and magnetoresistive read transducer assemblies. Examples of such devices are illustrated in European Patent Specifications Nos. 0276784 and 0314343, U.S Pat. No. 4,897,288 and International Patent Application No. WO91/18424, the entire disclosure of which are incorporated herein by reference.
Giant magneto resistive (GMR) materials of the iron chromium or cobalt copper type have been proposed for use in magnetic field sensors. The sensitivity of such devices incorporating GMR materials may be improved by the use of layered sandwich-type structures, but again the change in the electrical resistance at low magnetic fields is not enough for many applicaitons. In high fields at room temperature, however, a 60% .delta.R/R is achievable.
Other magneto resistive materials which have been suggested include cobalt silver, but this is a very fine grain material and inherently magnetically hard. It is only useful in devices for measuring high magnetic field strengths.
In Phys Rev Lett 71 (1993) 2331, the entire disclosure of which is incorporated herein by reference, there is described a GMR material comprising a mixed oxide of lanthanum, barium and manganese with which a .delta./R of 97% was achieved at room temperature in high fields.
None of the magnetic field sensors which have hitherto been proposed have achieved the combination of sensitivity, size and ease of manufacture necessary for many applications and any improvement in this respect would be hight desirable.